Color killer apparatus for television receivers



June 14, 1960 c. F. CHANDLER COLCR KILLER APPARATUS FOR TELEVISION RECEIVERS Filed Feb. 13, 1956 5 Sheets-Sheet 1 incas/ir June 14, 1960 c. F. CHANDLER 2,941,031

coLoR KILLER APPARATUS FOR TELEVISION REEIvERs med Feb. 1s, 1955 5 sheets-sheet 2 I I Il; l lll H' n, fr; l i/ g I i. .J i7" 22 @,J "W is EJ xZJ INVENTOR. 64Mo F, CHA/vm se 5 Sheets-Sheet 3 C. F. CHANDLER COLOR KILLER APPARATUS FOR TELEVISIGN RECEIVERS Filed Feb. 13, 1956 June 14, 1960 IN VEN TOR. (una F. few/vm i@ rrdP/V'y June 14, 1960 Filed Feb. 13,

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COLOR KILLER APPARATUS FOR TELEVISION RECEIVERS 1956 5 Sheets-Sheet 4 fla-74 irma/9:5

June 14, 1960 c. F. CHANDLER 2,941,031

COLOR KILLER APPARATUS FOR TELEVISION RECEIVERS Filed Feb. 13, 1956 5 Sheets-Sheet 5 /25 Nain/7L 50257' I I I I ld@ /000 l0, ddd M0, 000

5mn/4L frenan/(MMM vm ri) www waa aw r /35 VFL 75i apa @wir /ii 1N VEN TOR. (M20/.L E (H4/vm i@ BZW CoLon KILLER APPARATUS non retevision nncnrvnas Carroll F. Chandler, Palo Alto, Calif., assigner to Admiral Corporation, Chicago, El., a corporation o leias ware Filed Feb. 13, 1956, Ser. No. 565,092

16 Claims. (Cl. 178-5.4)

This invention relates to color television receivers that can also receive monochrome television signals, and in particular to improved color killers that provide better picture quality during reception of monochrome signals.

In compatiblecolor and monochrome television systems, as exemplied by current American television broadcasting standards, it is imperative that the color television receiver also be used to receive with substantially like eiiciency either color television signals or monochrome television signals, depending upon lwhich is transmitted. Color television receivers usually have separate luminance and chrorninance video channels for processing and transmitting to the picture tube luminance and chrominance components of the picture signal. Such receivers are well known to those skilled in the art, and have been described in numerous publications. For eX ample, a simplied description may be found in an article entitled The ABCs of Color Television by i. M. Barstow, published in the Proceedings of the lRE, November 1955, pages 1574 through 1579.

Monochrome or black-and-white television picture signals do not contain chrominance components. Consequently, the chrominance channel of a color television receiver serves no useful function during monochrome reception. But noise transmitted through the chrominance channel may seriously impair the quality of the monochrome picture. in fact, such noise is particularly objectionable during monochrome reception since it may produce colored noise in the monochrome picture. To eliminate this colored noise and thereby to improve the picture quality during monochrome reception, it is desirable to provide in color television receivers a color killer that automatically cuts oli the chrominance -channel while monochrome television signals are being received.

Color killers have been used prior to the present invention, but prior automatic color killers have been unreliable in operation. in particular, prior color killers have failed to operate reliably during reception of weak monochrome television signals having a low signal-tonoise ratio, which is the very time when cutting oft of the chrominance channel is most essential to satisfactory picture quality. Accordingly, an object of this invention is to provide an improved automatic color killer that operates reliably during reception of weak television signals having low signal-to-noise ratios, as well as during reception of stronger color television signals.

Heretofora automatic operation of a color killer has usually been controlled by a control voltage derived from any asymmetric tap in the color phase detector of the receiver that provides a negative voltage to cut oi the color killer responsive to the color bursts of received color television signals. Unfortunately, noise also produces a negative voltage at this tap. Even under conditions of good reception providing a strong color television signal, it has been found in practice that the color bursts may be as much as 20 decibels below normal in amplitude relative to other components of the picture signal. Con- 2,941,031 Fatented June 14, 1,960

sequently, to insure cutting oi of color killer during reception of such color television signals, the cut-olf level of the color killer must be adjusted to a fairly small negative value of the control voltage.

During the reception of weak monochrome television signals having a low signal-to-noise ratio, the automatic gain control of the receiver operates to increase the signal amplification for maintaining a satisfactory video signal level. The noise is also amplilied, and produces in the prior-art apparatus a large negative control voltage that cuts oi the color killer and permits the transmission of noise through the chrominance channel to the picture tube under these conditions, which is the time when such noise is most objectionable. Accordingly, another object of this invention is to provide improved lmeans for deriving a color killer control voltage that is substantially unresponsive to noise @ther-objects and advantages of the invention will -appear as the description proceeds.

Brieily stated, in accordance with certain aspects of this invention, a control voltage for the color killer is derived from a separate balanced or phase detector which also may be considered as a phase discriminator. Electric pulses of opposite polarity are supplied to two balanced input terminals of the detector in synchronism with the horizont ilyback pulses provided in the scanning system of the receiver. The detector is so arranged that these electric pulses in themselves have no substantial eiect upon the output voltage of the detector, but they do render the detector responsive to other signals during the occurrence of the pulses, while the detector is `nnresponsive to such other signals during intervals between the pulses.

Conventional demodulators of the receiver supply to the picture tube three color signals for producing a ytricolor picture presentation. One of these color signals, representing blue-ninus-luminance values and generally designated (EB-EY), includes demodulated color bursts of negative polarity during the reception of color television signals. This color signal is supplied to an unbalanced input terminal of the aforesaid detector, and in response to this signal the detector supplies a negative control voltage that cuts off the color killer.

ln a preferred form, the detector for supplying a color killer control voltage includes two capacitors connected to the balanced input terminals o the detector, two diode rectiers connected in series with like polarities between the two capacitors, and two resistors connected in series between the two capacitors. The demodulated color burst is supplied to the connection between the two rectiiiers, and the color killer control voltage appears at the connection between the two resistors.

rThe invention will be better understood from the following detailed description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims. ln the drawings:

Fig.V l is a schematic block diagram of a color television receiver including color killer apparatus embodying principles of this invention;

Fig. 2 is a group of curves representing electric waveforms that may be present at certain points in the circuit of a color television receiver;

Fig. 3 is a fragmentary simpliiied circuit diagramof a portion of the receiver shown in Fig. l, illustrating typif cal circuits of the rst chrominance amplifier, color killer and balanced detector;

Fig. 4 is a group of curves representing bias voltages supplied by the color killer to the rst chrominance am? plier;

Fig. 5 is a group of curves representing electric waveforms at various points in the color killer during reception of strong monochrome television signals;

Y 6 isaV group `of curvesrepresenting electn'c wavei forms in the colorkiller during reception of weak monochrome television signals;

Fig. 7 is a group of curves representing electric waveforms` inthe color killer during color television reception;

v Fig. 8 is a group ofrcurves representing relative Vvalues of the color killer control voltage in prior-art apparatus under various conditions of signal reception; and

Fig. 9 is a ygroup of curves representing relative values of the color killer control voltage in apparatus embody- 'ing theprent invention.

Y -It should be understood that the curves shown in the drawings are merely illustrative of principles to be eX, plained, andare not necessarily exact as toscale-and Vwaveform details. 'Y

Referring now to Fig. l of the-dIaWings, a color tele- -vision receiver includes a conventional antennaY 1, Ytuner V`2, LF. strip 3, luminance detector 4, delay line 5,'video Yamplitier 6, picture tube 7, automatic gain control or A.G.C. 8, audio system 9, loud-speakery 10, chromi- V nance detector 11, filter 12, first chrominance amplifier 13, ,second chrominance amplifier 14, demodulators 15, burst gate 16, color phase detector 17, reactancetube 18, subcarrier oscillator 19, syncseparatorZ, horizontal os'- cillator 21, horizontal output transformer 22', vertical oscillator l23, vertical output'itransformer 24, and a dection yoke 25. Such color television receivers are Well vknown to those skilled in the art, and thefundamental principles thereof are explained in the article The ABCs of Color Television hereinbefore identified.

Picture tube 7 may be of the well-known three-gun type having a separate electron gun for each of the three primary colors red, blue and green. Each electron gun has a cathode 26, a Vcontrol electrode 27, and one or more accelerating electrodesY 28. The Vaccelerating electrodes are connected to a suitable source of positive potential,

' indicated in the drawing by terminals 29,. OnV or near the large end face of the picture tube, there is a phosphor screen comprising dots or ships of three dilferent 'phosphors respectively adapted to emit light of the three primaryicolors when excitedrby electron'V bombardment.

An anode 30 may be an electron permeable metallic llm Vcoated on the back side of the phosphor screen. Anode V30 is-maintained at a high positive potential supplied by Vany suitable source indicated in the drawing by terminal 31. The picture tube also includes a shadow mask or some equivalent-device (not shown) so constructed and arranged that the electron beam from one of the guns strikes only phosphors adapted to emit red light, While the electron beam from a second one of the guns strikes only phosphorsY adaptedtoemit green light and the elec-V tron beam Vfrom the third one of the guns Yst rikesonly phosphors adapted to emit blue light.

A television signal received at antenna 1 is amplified and reduced in frequency by tuner 2, is further amplied Vby LF. strip 3, and is supplied to luminance detector Y4 and to chrominance detector 11. The audio portion of the received signal is detectedand amplified by audio system 9 and supplied to loud-speaker 10. Y Y

Luminance Vcomponents ofthe composite picturesignalare detected by luminance detector 4, and transmitted through delay line 5 and video amplifier 6 to the cathodes of the three electron guns. VAccording to conventional notaaion used Vby those skilled in the art, the luminance components are represented by the symbol (E'Y). VSince the luminance components are suppliedto the ncathodes Vrather than the control grids of the electron guns in the particular receiver illustrated, they areherein krepresented by the symbol (-E'Y). The luminance components of the picture signal are also supplied to A.G.C. 8 which controls the amplication of the tuner or the LR strip, vor both, to maintainv the VideoV signal level at a satisfactory Value. Y

The (-EY) signal includes horizontal and vertical V'synchronizing pulses. This signal is supplied to the sync separator 20 which supplies the horizontal synchronizing pulses to horizontal oscillator 21 and the vertical synchronizing pulses to vertical oscillator 23. Horizontal oscillator 21, which includes the horizontal output amplider, supplies to horizontal output transformer 22 current having an approximately sawtooth waveform synchronized With the horizontal synchronizing pulses. The voltage across transformer Z2 has a Waveform containing substantially rectangular pulses herein called horizontal yback pulses, synchronized with the horizontal synchro nizing pulses of the received picture signal. Taps on transformer 22 are connected to the horizontal dellection coils of yoke 25 to produce line scanning of the raster area by the three electron beams ofthe picture tube. Vertical oscillator 23, which includes the vertical output ampliner, suppliesV to vertical output transformer 24 current having an approximately'sawtooth waveform syn-v chronized with the vertical synchronizingpulses of the television signal. The secondary of transformer 24 is connected to the -vertical deection coils of yoke 25 to produce field scanningY ofthe raster area by the three electron beams.

Chrominance componentsy ofthe composite color picture signal are detected'by chrominance detector 11 and supplied to a lilter Y12j that vtransmits the chrominance Vwhich controls reactance tube 18 to regulate the phase Vof oscillations Vproduced by subcarrier-oscillator 19.

' Demodulators 15 are responsive to the amplitude of Ythe chrominance signal andV its phase relative to the oscillations produced by subcairier oscillator 19, and supply through leads 34,V 35 and 36 three color signals which, according to conventional terminology used by those skilled in the art, are respectively designated, (ER-EY), (EG-ny) and (E'B-E'Y). The (E'R-E'Y) sigan is supplied to the control electrode of the electron gun that produces the electron beam which strikes phosphors adapted to emit red lightythe (EG-EY) signal is supplied to the control electrode of the electron gun that produces the electron beam which strikes phosphors adapted to emit green light, and the (EB-EYl signal is supplied to the control electrode of the electron gun that produces the electron beam which strikes phosphors adapted to emit blueV light. As a result, the currents of the three electron beams are respectively functions of (ERL-(EG) and (EB), and light of the three Vprimary colors is produced in proper proportions vtoV synthesize the various colors of a color Vteleyision picture. Y Y Y ,y

Since luminance componentsrof Vthe composite picture signal are transmitted to the picture tube by'llunnnance Vdetector 4, delay line 5, Yand video amplifier 6, these components may collectively be referred: to as the luminance channel. Since chrominance components of the picture signal are transmitted to the picture tube by chrominance aaanosr y serves no useful purpose during monochrome reception. However, noise accompanying the received signal may be transmitted by both the luminance and the chrominance channels, and as a result of noise transmission through two channels the picture quality may be substantially poorer than it would be in a good monochrome television receiver during reception of the same television signals. The noise transmitted through the chrominance channel of a color television receiver during reception of monochrome television signals is especially objectionable since it may produce colored noise displayed on the screen of the picture tube.

The portions of the color television receiver shown in Fig. l that have been described thus far are conventional, and are typical of current color television receivers. It should be understood that various modilications in these portions of the receiver, including the substitution of diierent types of picture tubes, are immaterial to the present invention. The invention of this patent application relates to an improved color killer for cutting ott the chrominance channel of the receiver during monochrome reception to improve the picture quality by preventing the transmission of noise to the picture tube through the chrominance channel.

During reception of monochrome television signals, color killer 37 supplies through lead 38 to lirst chrominance amplilier 13 a negative bias voltage that cuts oft the lirst chrominance amplifier, except during horizontal yback as hereinafter explained, and thereby cuts off the chrominance channel to prevent the transmission of noise through this channel to the picture tube. For purposes hereinafter explained, electric pulses of opposite polarity synchronized with the horizontal tiyback pulses are supplied to the color killer through leads 39 and 40. These pulses may be derived-from secondaries 41 and 42 of horizontal output transformer 22. Preferably secondaries 41 and 42 are connected in series, and the connection between the two secondaries grounded, as shown. Secondary 42 may also supply through lead 43 pulses for operating burst gate 16.

During reception of color television signals, color killer 37 is cut olf by a negative control voltage supplied through lead 44.. It has been common practice heretofore to derive this control voltage from 'an asymmetric tap of color phase detector 17, but in this case noise has an appreciable etect upon the magnitude of the control voltage, especially during reception of weak monochrome television signals having a low signal-to-noise ratio. Consequently, prior color killers have been unreliable in operation and have failed to cut oit the chrominance channel during reception of weak monochrome television signals when noise transmitted through the chrominance channel is most objectionable.

In accordance with the present invention, the control voltage supplied to the color killer through lead 44 is provided by a separate balanced or phase detector 45 which may also be considered as a phase discriminator. Electric pulses of opposite polarity synchronized with the horizontal yback pulses are supplied to a pair of balanced input terminals 46 and 47 of detector 45 by connections to secondaries 41 and 42 of the horizontal output transformer. A color signal that includes demodulated color bursts of negative polarity is supplied to an unbalanced input terminal 48 of detector 45 through a lead 49 from demodulators 15. Detector 45 provides at its output terminal 5t) a voltage having a negative value that is related to the amplitude of the demodulated color bursts supplied to input terminal 48. Noise has little elect upon the value of the voltage at terminal 50. Lead 44 is connected to output terminal 50 of the detector, and supplies the detector output voltage to the color killer as a control voltage to cut oif the color killer during reception of color television signals.

According to current American color television broadcasting standards, the color burst has a phase angle of degrees relative to reference phase, while the (EB) signal has a phase angle of 347 degrees relative to reference phase. Thus the color burst and the (EB) signals are almost in phase opposition. Consequently, the (BB-EY) signal supplied to lead 36 by demodulators 15 includes demodulated color bursts of negative polarity. Accordingly, in color television receivers designed for the reception of color television signals conforming to American broadcasting standards or equivalent standards, lead 49 preferably is connected to lead 36 so that the (EB-EY) signal is transmitted to unbalanced input terminal 48 of detector 45. In television systems conforming to other standards, it may be more desirable to supply one of the other two color signals to terminal 43 of the detector, or other means may be provided for supplying demodulated color bursts to terminal 4S.

Before proceeding with a more detailed description of the color killer apparatus, it may be helpful to consider briefly the curves shown in Fig. 2, which illustrate certain waveforms found at various points in a color television receiver circuit. Solid curve 51 represents a portion of a typical color television composite picture signal produced during transmission of a color bar pattern consisting of colored vertical bars arranged in the sequence green, yellow, red, magenta, blue, cyan and green. The composite picture signal includes horizontal synchronizing pulses 52 and 53 transmitted at a repetition rate corresponding to the line scanning frequency. Color bursts 54 and 5S are transmitted on the back porch of the horizontal synchronizing pulses for controlling the phase of oscillations produced by the subcarrier oscillator of the receiver. The color bursts are oscillations at the chrominance subcarrier frequency with a phase angle of 180 degrees relative to reference phase. The luminance components (E'Y) of the picture signal are represented by broken line 56. Chrominance components of the picture signal appear as oscillations at the chrominance subcarrier frequency modulated in amplitude and phase in accordance with the saturation and hue of the picture element. In the drawing, the number of chrominance subcarr'ier oscillations has been reduced for clarity. Portion 57 of the picture signal represents a green color bar, portion 58 represents a yellow color bar, portion 59 represents ak red color bar, portion 60 represents a magenta color bar, portion 61 represents a blue color bar, portion 62 represents a cyan color bar, and portion 63 represents another green color bar.

Curve 64 of Fig. 2 represents the (EB-EY) signal supplied to leads 36 and 49 by demodulators 15. For purposes of the present invention, the important thing to note about curve 64 is that the demodulated color bursts appear as negative pulses 65 and 66.

Curve 67 represents a voltage signal supplied by horizontal oscillator 21 across horizontal output transformer 22. This signal includes yback pulses 68 and 69 that are synchronized with the horizontal synchronizing pulses 52 and 53. When a voltage having a Waveform similar to curve 67 is applied to the horizontal deflection coil of deflection yoke 25, a sawtooth waveform current is produced through the horizontal deection coil to provide line scanning of the raster area on the screen of the picture tube. Since these waveforms are generated by conventional apparatus well known to those skilled in the art, any further description thereof would be superfluous.

Reference is now made to Fig. 3 of the drawings, which is a simpliiied circuit diagram of the iirst chrominance ampliiier 13, the color killer 37, and the balanced detector 45. Amplifier 13 may be a conventional ampliier stage incorporating an electron discharge device such as pentode vacuum tube 70. The rst grid or control electrode of tube 70 is connected to lead 32 through a capacitor 71, and is connected to lead 38 through a bias circuit consisting of inductors 72 and 73, .a grid leak resistor 74, and a capacitor 75, connected as shown. Chrominance components of the picture signal are transmitted from lead .32

through capacitor 71 to tube 70, while the bias circuit 7.2.-.75 Yacts as a filter to prevent transmission of the chrominance components ,to lead 38. On the other hand,

. 'connected to ground or its circuit equivalent, and anode 79 is connected to leadS through a capacitor 80. A

Y potentiometer 81 Yancla resistorr82 are. connected in series betweenground and lead 39. Potentiometertl has an adjustable tap that is connected through aresistor 83 .to anode .79..Y A resistor 84 isrconnected between anode 79 and a source of positive potential, about 200 volts, for example, represented in the drawing by terminal 85. A capacitor 86 and aresistor 87 are connected ,in series as shown between control electrode 7Y8Yand lead 40. A resistor 8S` is cor'inectedA betweengroundand the connection between capacitor 86 and resistor -8.7, asA shown. A resistor 89 is `connected between control electrode 78 and lead 44. A resistor 90'is connected between control electrode 78 and a source of positive potential, about 200 volts, for example, represented inthe drawingby terminal 91. Y

Color killer 37 also includes a clamp circuit compris- Ying a diode rectifier which may be a triode vacuum tube 92 having its controlelectrode 93 connecteddirectly Vto its anode V94.V The cathode 95 of't'ube 92 is connected to ground or its circuit equivalent through a'resistor 96, andV is connected through a resistor V97 to a source of negatube during-the reception of color television signals, and can be adjusted by' adjusting the position of the tap of potentiometer 81. Current ilowing from theY source of positive potential at terminal 91 through resistors 102 and 99vto lead 38 insures that the peaksof the positive pulses will not fall below the level established by tube 92, and prevents the negative bias suppliednto Ythe amplifier tube 70 from ybecoming larger in magnitude than is desired.

Now assume that the control voltage supplied tothe color killer through lead 44 is substantially zero, asis'the case during reception of monochrome television signals. Under these conditions, tube 76 is conductive during intervals between the electric pulses supplied to leads 39 and 40 by the horizontal output transformer, and electric Acurrent'flows from the positive potential source at terminal .through resistor 84 and tube 76. In synchronism with Vthe horizontalV yback pulses, secondary 42 of the horizontal output transformer supplies anegative electric pulse through lead 40, resistor 87 and capacitor 86to the control electrode Y78 of tube .76, and .these negative pulses Aabruptly cut off tube 76. When tube 76 Yis cut olf, its

` anode potential rises, and as a result relatively large positive pulses are transmitted through capacitor 80 to lead The clamp tential of lead 38 at the positive peaks of the pulses supplied thereto, so that between these pulses the bias voltage supplied through lead 38 to the control electrode Yof tube 70 has a relatively large negative value, as is 3oV indicated by curve 105 of Fig. 4. Consequently, tube 70 is cut 0E duringrintervals between the yback pulses, and no noise or other .signals can be transmitted by the chrominance channel during the active portion of a horitive potential, .about --l2gvolts, for example, represented f inthe drawing by terminal\98. Control electrode 93 `and anode 94 Vare connected to ground through-a resistor 99 and a capacitor Y100 is connected in seriesas shown. The

connection between resistor `99 vand capacitor 100 isV connected to lead 44 through a resistor 101, and is connected to terminal 91 through a resistor 102. c

Operation of the color killer may best be understood by reference' to the curvesshownin Fig. 4. 1n Fig. 4, horizontal solid line 1.03 represents zero potential, ysolid curve 104 represents the waveform of a bias voltage supplied by the colorkiller to lead 38 during the reception of color television signals, andbrokenV curve 105 represents the waveform of the bias voltage supplied by the color Vkiller to lead'Y 38 during reception of monochrome television signals. j Y l" Referring now to YFigs. V3 and 4,V assume thata sutilciently negative control voltage is supplied through lead 44 to control electrode 78 to cut off tube 76, 'as is the Ycase during reception of color television pictures, as will hereinafter be more fully explained. In synchronism with the horizontal ilyback pulses, secondary 410i the horizontal output'trans'former supplies positive electric pulses to the colorv killer through lead 39 and supplies negative electric Vpulses to the'colorkiller through lead 4i). 'Sinee'the tube The positive pulses are transmitting throughresistor 82,

a portion of potentiometer' 81, resistor 83 'capacitor 80 to lead 38. However, the maximum positive potential including tube 92, since tube 92 conducts current Whenever its anode becomes more positive thanits cathode.

Y Consequently,` the peaks of the positive pulses suppliedto 95h95. me @las '.Vliage SlPPlQdlQ-the .Control-electrode. rof

. l Y65 that lead Y38 can haveis 'established by the clampcir'cu'it zontalscanning line while monochrome television signals are being received. During the flyback portion of a horizontal -Y scanning line,Y however, YYwhich corresponds to arpositive peak of the pulses supplied Vto lead 3 8, Atube 70 is conductive so that Ycolortnrsts can be transmitted through the chrominance channel whenever the receiver is Vtuned* to receive a color television signal.

The control voltage is supplied to lead 44 by balanced Adetector 45.3 Again'referring to Fig. 3, the'balanceddetector includes a series resistor 106 and ashunt capacitor 107, forming a first Yintegrating circuit, connected to its Yinp1 1tterm.inal 46, and a seriesresistor 108'and`a shunt capacitor Y109,'forming a second integrating circuit, connected to its input terminal 47. A' capacitor 110 is connected in series with resistor 106, and a capacitor 111 is connected in series with resistor 108, as shown. Two halfwave rectifiers, whichY may be diode vacuum tubes 112 and 113, are connected in series with like polarities between capacitors and 111. Two load resistors'114 and 115 are also connected in series between capacitors 110 and 111, and are in parallel with rectiers 112 and 113. A series capacitor 116 a'nd a shunt resistor 117, forming a differentiating circuit, are connected to input terminal 48, as shown. The connection between rectiers 112 and 113 vis connected to capacitor 116 and the connection between resistors 114 and 115v is connected to output terminal 50.

` In synchronism with the'horizontal flybackpulses and therefore in substantial synchronism with the color bursts transmitted on the back porch of the horizontal synchronizing pulses, secondary v41 of the horizontal'output transformer 'supplies positive electric pulses to input terminal 46 of the 'balanced detector, and simultaneously v therewith secondary', 42 supplies negative -electric-pulses .the .negative pulses may. have an-amplitude vofvaboutV 10Q volts peak to peak.v .This-can -be *accomplishedV by providcircuit including tube 92 establishes the popositive pulses are partially integrated'by integrating'circuit 1136-167, which may have a time Vconstant of about 3 microseconds. Similarly, the negative pulses are partially integrated by integrating circuit S--109, which may also have a time constant of about 3 microseconds. The partially integrated positive pulses are supplied to capacitor 1111, and the partially integrated negative pulses are supplied to capacitor 111.

Rectiiers 112 and 113 are of conductive polarity with respect to the pulses supplied to terminals 46 and 47, and conduct current during these pulses. This charges capacitors 110 and 111 so that, during intervals between the pulses, the connection 118 between capacitor 110 and rectifier 112 is maintained at a negative potential, while the connection 119 between capacitor 111 and rectier 113 is maintained at a positive potential. Capacitors 116 and 111 discharge slightly through resistors 114 and 115 during intervals between the pulses, and the charge on the capacitors is restored bythe conduction of current through rectiers 112 and 113 during positive peaks of the pulses. By Way of example, capacitors 110 and 111 may each have a value of about .0l microfarad, and resistors 114 and 115 may each have a value of about l megohm, so that the time constant of the capacitor discharge between pulses is in the order of 100th of a second. Since the line scanning period is only about 63.5 microseconds, the discharge of capacitors 110 and 111 between pulses is quite small in amount.

The potential of connection 120 between rectiiers 112 and 113 depends upon the signal supplied to unbalanced input terminal 48 of the detector through lead 49. When no signal is supplied to input terminal 48, the potential of lead 120 is substantially zero since this lead is connected to ground through resistor 117 and substantially equal amounts of current pass through the rectiers 112 and 113 during the pulses supplied to terminals 46 and 47. During reception of color television signals, the (E'B-EY) signal is supplied to terminal 4S through lead 49, and this signal is partially diierentiated by the differentiating circuit consisting of capacitor 116 and resistor 117, which may, for example, have a time constant of about 2 microseconds. The partially diterentiated (EB-EY) signal is supplied to and establishes the potential of connection 120 between the two rectiers.

Now assume that a strong monochrome television signal having a high signal-to-noise ratio is being received. Since a monochrome television signal contains no chrominance components, the (EB--EY) signal has zero amplitude, and the potential of connection 120 is substantially zero and constant. Operation of balanced detector 45 under these conditions can best be understood by reference to the curves illustrated in Fig. 5. In Fig. 5, horizontal solid line 121 represents the potential of connection 120, which is substantially zero. Broken curve 122 represents the potential of connection 118, and illustrates the positive electric pulses supplied in synchronism with the horizontal iiyback pulses by transformer secondary 41 through integrating circuit 10G- 107 and capacitor 110 to connection 118. Broken curve 123 represents the potential of connection 119, and illustrates the negative electric pulses supplied in synchronism with the horizontal yback pulses by transformer secondary 42 through integrating circuit 108-109 and capacitor 111 to connection 119.

It will be noted that the amplitude of the positive pulses is somewhat greater than the amplitude of the negative pulses, due to the fact that there are more turns in secondary 41 than in secondary 42. The principal eiect of the integrating circuit is to round ofi the corners of the pulses and accentuate the trailing edges of the pulses relative to the leading edges of the pulses, as is shown in Fig.- 5. The slight slope of curves 122 and 123 between pulses represents the partial discharge of the capacitors 110 and 111 through resistors 114 and 115.

1G u f Solid curve `124v of Fig. 5 representsthepotential. at output terminal 5G of. the balanced detector. Because resistors 114V and 115 constitute a voltage divider between connections 118 and 119, curve 124 lies-almost midway between curves 122 and 123, but is slightly posi-v tive with respect to the true midpoint because of the small current tiowing to terminal 50 from the positive voltage supply at terminal 91 through resistors 90 and 89. Furthermore, control. electrode 78 is slightly more positive than detector terminal 50 because of a small voltage drop across resistor 89. However, the true midpoint between the potentials of leads 118 and 119 is a somewhat negative potential because of the difference in amplitude between the positive and negative pulses supplied to terminals 46 and 47. These factors are mutually compensating so that the potential ofcontrol electrode i8 is substantially zero during monochrome reception. In other words, because secondary 41v has more turns than secondary 42, the control voltage at terminal 50 is somewhat negative in the absence or substantial absence of an (EJB-EY) color signal, sothat the bias ofV tube 76 may conveniently be adjusted by selecting appropriate values of resistors S9 and 9G to set Vthe color burst level at which the color killer will operate.

Whenever the potential of connection 11S, represented. by curve 122, is more positive than the potential at connection 120, represented by curve 121, current flows. through diode rectier 112. Since this current chargesy capacitor 116 and makes the potential at lead 118 moreV negative, the amount of current that rectifier 112 will conduct during each positive pulse is just sucient tol replace the small amount of charge lost by capacitor 11i) between pulses by current through the resistors 114- and 115. As a result, the potential of lead 118 is main-v tained at sucha value that rectifier 112 is conductive only during the most positive or peak portions of the positive electric pulses, and the potential of the positivey peak is controlled by the potential of connection 120l during these positive peaks. This in turn, in combina-l tion with lthe pulse amplitude, controls the value of the` negative potential at connection 118 between the posi-- tive pulses.

Similarly, diode rectifier 113 conducts current only when the potential of connection 119, represented by curve 123, is more negative than the potential at connection 121i, represented by curve 121, so lthat the po-Y tential of connection 120 during negative peaks of the negative pulses, in combination with the amplitude of the negative pulses, controls the value of the positivepotential at connection 119 between the pulses.

It is thus apparent that the potential at output termi-- nal 50 is chiey controlled'by the value of the potentialr supplied to connection 120 through input terminal 48, and that similar signals of opposite polarity suppliedl simultaneously to input terminals 46 and 47 have little.v er'iect upon thepotential of output terminal 50. Consee quently, terminals 46 andy 47 may be referred to as. balanced input terminals of the detector, and terminali. may be referred to as an unbalanced input terminal. The potential at output terminal 50 of the balanced dev tector is supplied through lead 44 and resistor 89 to thecontrol electrode of tube 76 in the color killer, and acts as a control voltage for cutting olf the color killer during reception of color television signals.

The negative magnitude off the voltage at terminal'l) varies in almost exactly -direct proportion to the amplitude of the color burst. When the received signal has: large chrominance components including a large-ampli tude color burst, a relatively large negative voltage isprovided at terminal 5t). A portion of this negative voltage is transmitted through resistors 101 and 99 tothe bias circuit or" tube 70, and reduces the amplification provided by tube 7?. A chrominance-component automatic gain control is thus provided. The value of re sistor 102 sets theoperatingrange'of the grid bias volt-A age for tube 70.

wenn

l DuringV monochrome reception, the value of the controlf voltage is substantially zero, as hereinbefore eX- plained, andY tube 76 isV conductive during intervals between horizontal yback pulses. In synchronism with i the flyback pulses, tube 76 is cut oi by negative pulses supplied to its control grid through lead 40. Asa result,'the chrominance channel of the receiver is cut oi during intervals between the yback pulses, and transmits signals only dun'ng horizontal iiyback. i

Now assume that a -weak monochrome television signal having a small signal-to-noise ratio is being received. Operation of balanced detector 45 under these conditions can best be understood by reference to the curves shown in Fig. 6. Solid curve 121' Irepresents the potential of connection 120 between the two diode rectiers, broken curve 122 represents the potential of connection 118, broken curve 123 represents the potential of connection 119, and solid curve 124' represents the control voltage provided at output terminal 50 of the detector. Y During horizontal flyback, tube 70 of the first chrominance amplifier is conductive, and noise is transmitted through the chrominance channel of the receiver. This noise has no appreciableeiect upon the picture quality, since the picture tube is blanked during ilyback. However, noise bursts are transmitted through demodulators 15 during yback, and these noise bursts are transmitted through lead 49 to connection 120 of the balanced detec-V tor. Y Consequently, the potential at connection 120 ofthe detector hasa waveform such as that represented by curve 121'. j s t j The potential at connection 118 of the detector, represented Vfby curve 122', is now established by the negative Ypeaks of the noise bursts, Vand the potential at connection'119, `represen-ted `by curve 123 is established by the positive peaks of theV noisefbursts. As a result, curves 122,jand 123' of Fig. 6 are somewhat further apart than curves 122 and 123 of Fig` 5, but this displacement of curves v122' and 123 is substantially symmetrical with respect to zero potential, and consequently the value of the control voltage at output terminal 50 of the detector, represented by curve 124', remains substantially zero. Ilt is thus apparent that the value of the control voltage is relatively independent of noise transmitted through the chrominance channel of the receiver. Be-

cause of' thisfact, the color killer' operatesV reliably to cut offV the chrominancefchannel during reception of weak monochrome signals having smallvsignal-to-noise ratios, as WellY as during reception ofrstrong monochrome television signals having large signal-to-noise ratios.

Operation ofk balanced detector 45 during reception of color television signals can best be understood by reference to the curves of Fig. 7. During color recep- Y tion, a (EB-EY) signal, which may have a waveform similar to that represented by curve 64 of Fig. 2, is transmitted through-lead 49 to unbalanced input terminal 48 of the balanced detector. This signalis partially dierentiated by the diierentiating circuit 1164-117, and the resulting signal supplied to connection 120 of the balanced detector may have a waveform such as that represented the potential of connection'llS ofthe detector, curve 123l represents thepotential of connection 119 of the detector, and curve 124" represents the control voltage Y by curve 121" 'oftFign V7. Curve 122" of Fig. 7 represents 1-2 voltage cuts. oitube76 ofthe G9101Y killer. Nowv the color killer is ineffective to rcut ofztube 70 of the iirst chrominance amplien `and. the chrominance channel transmits signalsV continuously during color television reception.. During intervals between the horizontal yback pulses, both of the diode rectiliers 112 and 113 are cutoi by large reverse voltages, and the detector is unresponsive to the potential at connection 120. In effect, therefore, the detector is responsive only to theY demodulated color burst portions of the (EB-E'Y) signal, and the amplitude of the color bursts determines the negative value of the control voltage.

Advantages of the present invention can best be appreciated by reference Yto the curves shown in Figs. 8 and 9. Fig. S is a group of curves showing relative values under various conditions of a control voltage derived from an asymmetric tap of color phase detector 17, which according to prior practice was used to control the operation of the color killer 37. Fig. 9 Ais a similar grouppof curves showing relative values under various conditions of the controlV voltage supplied Vto the color killer by balanced detector 45 in accordance with the present invention. i

Referring now to Fig. 8, curve 127 represents the relative value of a control voltage derived from an asymmetric tap of color phase detector 17 as a function of signal Ystrength'of a color television signal received at antenna 1 when the amplitude of the color bursts hasthe normal or correct value relative to other componentsof the compositeY picturersignals. ,Howeven in actual practice it vhas been found that'the amplitude of the color bursts maybe as much as 20 decibels belowtthe normal value, even under conditions of goodreception. Accordingly, curve 12S represents the value of the control voltage whenthe. amplitude of the color bursts is 6 decibels below normal, andpcurve i129 represents the value of the control voltageV when the amplitude of lthe color burst is l0 decibels below the normal value. Curve 130 represents the value of the control voltage. during reception of a monochrometelevision signal. i

' Because the control voltage developed at an asymmetric tappf color phase detectorf17 Vis influenced by noise accompanying the received signal, a relatively large control voltagev is Ydeveloped during reception of weak'monochrome signals having a small signal-to-noise ratio, which is represented. in Fig. Sby the relatively Alarge negative value Yofvcrurve 130 at the left sideof the figure.` flo in; sure that the color killer will be cut oi duringnreception of Ycolor television signals having color bursts Withjampli- .tudes considerably less `than normal,Y the controljvoltage value at which .the color killer is out oi rnust be set at a broken lineV 131 of Fig.' 8.

Under these conditions, it is evident that the color killer will be cut oi during reception of. monochrome television signals when the strengthy of the signal receivedat antenna 1 is less vthan about 1000 'microvoltsY Consequently, automatic color killer apparatus heretofore, employed has been intfective to cutroii the chominance channel 'ofthe receiverduring reception of weak 'monochrome signals having small signal-tonoise ratios,n'hich is .justthe time when noise transmitted through ythe chrominance channel is most objectionable. Y Y Y l Fig. 9 represents relative values'under variousconditions of the control YoltageY supplied at theoutputterininal 50 of balanced detector 45. Curve 131 represents relaf tive values of the control voltage during Vreception of4 color television, signals having color bursts with anormal amf plitude relative to other components of the composite picture signal., Curve 132 represents relative .valnesof the Vcontrol signal` when then amplitude of the VYcolor bursts is rdecibels below normal, curve 133 represents relative values` of the control voltage when thekamp'litude ot the Y color burst is l0 decibels below4 normal,V and, curve 134 lto a control voltage value such as that represented by broken line i316 of Fig. 9. This adjustment may be made, for example, by appropriate choice of values for resistors 89 and 9% and other circuit elements of the receiver. It will be noted from Fig. 9 that the control voltage during monochrome reception, represented in Fig. 9 15 by curve l35, is always more positive than the cut-olf level represented by broken line 136. Consequently, the color killer is eective to cut oi the chrominance channel during monochrome reception even though the received monochrome television signal may be of low strength and have a small signal-to-noise ratio.

It should be understood that this invention in its broader aspects is not limited to the specific embodiment herein illustrated and described, and that the following clairns are intended to cover all changes and modifications that do not depart from the true spirit and scope of the invention. Although reference has herein been made to American color television broadcastingy standards, such reference is by way of illustration only, and it will be understood that the invention is also applicable to color television receivers designed for television systems conforming to other standards.

What is claimed is:

l. In a color television receiver having a chrominance channel for transmitting chrominance signals and means for providing demodulated color bursts during reception of color television signals, the combination comprising a color killer circuit for cutting od said chrominance channel during reception of monochrome television signals, a balanced detector means to apply pulses of oppo 40 site polarity which are synchronized with the line scanning requency to the opposite parts of the balanced detector, means to supply said demodulated color bursts also to the balanced detector so that opposite polarity pulses are combined with the demodulated color bursts,

and means to apply biased signals developed from the comparison and combination of signals to render the color killer circuit inoperative during periods of reception of the color burst signals.

2. In a color television receiver having a chrominance 5D amplilier and means for providing demodulated color bursts during reception of color television signals, the combination comprising a color killer circuit for supplying a bias voltage to cut off said chrominance amplifier during reception of monochrome television signals, a

balanced detector, means to apply pulses of opposite polarity which are synchronized with the line scanning frequency to the two parts of balanced detector, means to supply to said demodulated color bursts for supplying a control voltage to out oli said color killer circuit during reception of color television signals.

3. In a color television receiver having a chrominance amplifier and a demodulator for supplying during reception of color television signals a (Egfr-BY) signal that is followed by demodulated color bursts, apparatus com-` prising a color killer circuit for supplying a bias voltage to cut ott said chrominance amplicr during reception of monochrome television signals, a balanced detector responsive to the color burst portions only or said (EB-EY) signal for supplying a control voltage to cut off said color killer circuit during reception ofv color television signals.

4. In a color television receiver having a chrominance channel for transmitting chrominance singals, a color killer circuit for cutting olf said chrominance channel during reception of monochrome television signals, comprising a detector for supplying a control voltage to bias said chrominance channel to an operative state during reception of color television signals, said detector having a pair of balanced input terminals and an unbalanced input terminal, means for supplying demodulated colorl bursts to said unbalanced input terminal during reception of color television signals, means for simultaneously supplying electric pulses of opposite polarity synchronously with the line scanning frequency to said balanced input terminals.

5. In a color television receivery having a chrominance channel for transmitting chrominance signals, means for providing horizontal yback pulses, and a demodulator for providing demodulated color bursts during reception of color television signals, a color killer circuit for cutting 01T said chrominance channel during reception of monochrome television signals, comprising a detector having two balanced input terminals and one unbalanced input terminal, means for supplying a control voltage to cut oi said color killer circuitY during reception of color television signals, connections for supplying said demodulated color bursts to said unbalanced input terminal, and means for supplying electric pulses of opposite polarity to said balanced input terminals in synchronism with said horizontal yback pulses so that the pulsesV applied to the balanced input terminals are compared with the demodulated color bursts and during reception of color signals the control voltage is developed.

6. In a color television receiver, a chrominance channel for transmitting chrominance signals, means for providing horizontal yback pulses, a demodulator for providing demodulated color bursts during reception of color television signals, a color killer means for cutting o said chrominance channel duringv reception of monochrome television signals, two capacitors, two halt-wave rectiiers connected in series with like polarities between said two capacitors, two resistors connected in series between said two capacitors, means for supplying electric pulses of opposite polarity to said two capacitors in synchronism with said horizontal flyback pulses, said rectiers being of conductive polarity with respect to said electric pulses, connections for supplyingV said demodulated color bursts between said two rectiiiers, whereby there is produced between saidY two resistors a control voltage related in value to the amplitude of said demodulated color bursts, and connections for supplying said control voltage to said color killer means to render the color killer means inoperative to cut olf the chrominance channel duringl reception of color television signals.

7. ln a color television receiver, a chrominance amplifier, a demodulator for supplying during reception or color television signals a color signal that includesY demodulated color bursts of negative polarity, a transformer, means for supplying horizontal iiyback pulses to said transformer, a color killer means for supplying a bias voltage to cut ott said chrominance amplifier during reception of monochrome television signals, two capacitors, two diode rectiers connected in series with like polarity between said two capacitors, two resistors connected in series between said two capacitors, circuit means connecting said two capacitors to points on said transformer whereat maximum voltage diderences appear so that electric pulses of opposite polarity are supplied to respective ones of said capacitors in synchronism with said horizontal llyback pulses, said rectiiers being of conductive polarity relative to said electrical pulses, circuit means for supplying said color signal between said two rectiers, whereby the ilyback pulses are compared with the demodulated color bursts and there is produced between said two resistors a control voltage related in value to the amplitude of said demodulated color bursts, and circuit means for supplying said control vvoltage to said color killer means to cut olf the color killer means during reception of color television signals.

'8. In a color and monochrome television receiver, a chrominance channel, means for supplying demodulated color bursts during reception of color television signals, meansV for developing horizontal flyback pulses during Y receipt of television signals, a color killer means for cutting oli said chrominance channel during reception of monochrome television signals, two integrating circuits,

two capacitors connected to respective ones of said Vintude of said demodulated colorvbursts, and means Vfor supplying said control voltage to said color killer means to'cut ot the color` killer means-during reception of color television signals. f Y

. 9. Apparatus as deiined inclam 8, inY whichA each of said integrating circuits consists of-,a series resistor andV a shunt capacitor. v

10. In a television receiver for color and monochrome, a chrominance channel, means for producing from the chrominance channel demodulated color burstsrduring re#` ception of color television signals, means Yfor producing from the received television signals horizontal flyback pulses, means for cutting oi said Ychrominance channel during reception of monochrome television signals, two capacitors, Y.two half-wave rectiers connected in series with like polarities between said two capacitors, two resistors connected in series between said two capacitors, means for supplying electric pulses of opposite polarity to respective ones of said capacitorsY in synchronism 4with said ilybackV pulses, a diierentiatiug circuit, meansV for supplying said demodulated color bursts through said differentiating circuit to the connection between said two rectiers, whereby there is developed at the connection between said two resistors a control voltage related in'value to the amplitude of said demodulated color bursts, and means for supplying said control voltage toA said color killer to cut off the color killer means during receptio of color television signals. t,

11. Apparatus as defined in claim l0, in wbichsaid differentiating circuit consists of a series capacitor and a shunt resistor. v Y

=12. In a television receiver for colorV and monochrome, Y

Ya color killer circuit comprising, two integrating circuits each including of aseries resistor andra shuntcapacitor,Y a differentiating circuit including a rst series capacitorY and a shunt'resistor, second and third series capacitors connected.A in series with respective ones of said series resistors, two diode rectifiers connected in series with like polarity between said second and third series capacitors, said lirst series capacitorbeing connected tothe connection between said rectiiiers, two load resistors connected in series between said second and Vthird series capacitors, means for supplying demodulated color bursts to said first series capacitor, means for supplying to respective ones of said series resistors electric pulses of opposite polarity substantially in synchronism with said color bursts, said rectiliers being of conductive polarity with respect to said electric pulses, whereby there is provided at the connection between said load resistors control voltage related in value to the amplitude of said color bursts and circuit means responsive `to said control voltage to cut oi the color killer circuit during the reception oftcolor television signals, p Y Y Y 13'.v In a color television receiver having a chrominance channel for .transmitting chrominance signals, means for providing horizontal yback pulses, a demodulator for providing demodulated color bursts during reception of color television signals, a color killer circuit operative to cut oi said chrominance channel during reception of monochrome television signals, two capacitors, two halfwave rectiiers connected in series with like polarities between said two capacitors, rst and second resistors connected in series between said Itwo capacitors, a source of positive potential, a third resistor connected betweentsaid source of positive potential and a connection between said iirst and second resistors, means for supplying electm'c pulses of opposite polarity to said two capacitors in synchronism with said horizontal yback pulses, said rectiiers being of conductive polarity with respect to said electric pulses, connections for supplying said demodulated color bursts to the connection between said rectiers, whereby there is produced at the connection between said first and second resistors a control voltage related in value to the amplitude of said color bursts, and connections for supplying said control voltage to said Vcolor killer circuit to cutV ot the color killer effect and render the chrominance channel operative during reception of color television signals.

14. Apparatus as defined in claim 13, in which said pulses ofiopposite polarity consist of simultaneous positive and negative pulses, said positive pulses being of larger amplitude than said negative pulses.

`15. ln a color television receiver having a chrominance channel, bias connections for cutting oft said chrominance channel, means for supplyingl demodulated color bursts during reception of color television signals, Yand means for supplying horizontal flyback pulses, apparatus comprising an electron discharge device having a cathode and a control electrode and an anode, means forY supplying negative voltage pulses to said control electrode in synchronism with said flyback pulses, whereby positive electric 4pulses may befprovided at said anode, circuit means for supplying said positive pulses to said bias connections, a clamp circuit for establishing the most positive potential that said bias connections can have, whereby a negative bias voltage is supplied to said bias connections for cutting Voifr said' chrominance channel between said positive pulses, a balanced detector` for supplying a control voltage having a negative value related to `the amplitude of said demodulated color bursts, and circuitY means for supplying said control voltage to fsaid control electrode, whereby said electron discharge device is cut off during reception of color television signals,

16. In a color television receiver having a chrominance amplifier, a bias circuit for said amplifier, a demodulator for supplying during reception or" color television signals a (E'B-EY) color signal that includes [demodulated Vcolor bursts of negative Vpolarity,'a transformer, and

means for supplying horizontal yback pulses-to said transformer, apparatusv comprising two capacitors, circuit connections between said transformer and said capacitors for supplying electric pulses of opposite polarity to respective ones of said two capacitors in synchronism with said flyback pulses, two vdiode rectiiers connected in series between said two capacitors, said rectiers being` of conductive polarity with respect to said electric pulses, two resistors connected in series between said two capacitors, circuit connections Vfor supplying said (E'B-EY) signal to the connection between said rectiers,'whereby there is provided at the connectionl between said resistors a control voltage having a negative value directly related to the amplitude of said demodulated color bursts', an electron discharge device having a cathode and a control electrode and an anode, connections for supplying said control voltage to saidr'control electrode, whereby said electron discharge device is conductive during reception of monochrome television Vs ignels and is cut oil during reception of color television signals, meansA for supplyingto said electron dlscharge 17 18 Y device electric pulses synchronized with said yback amplifier is cut o during reception of monochrome pulses so that positive pulses are produced at said anode pictures. when said device is conductive, circuit means connecting said anode to said bias circuit, and a clamp circuit for References Cited in the me 0f fhlS P9tem establishing the most positive potential that Said bias 5 RCA service Data 1954, T13, Model zl-Cr-ss; circuit can have, whereby a negative potential is applied November 24, 1954 (pages 25 2g) to said bias circuit between said positive pulses and said 

